Continued development of mirrors and mirror assemblies especially designed for use in vehicles has recently given rise to assemblies in which lightweight mirror cases house reflective mirror elements. Such lightweight assemblies greatly reduce the effects of image blurring vibration transmitted thereto when mounted in a vehicle. One such mirror assembly is disclosed in U.S. patent invented by Arthur W. Kurz, Jr. and Harold R. Wilson and entitled MIRROR CASE AND, and assigned to the same assignee as the present invention. That rearview mirror includes a hollow case formed from a thermoplastic material with a reflective mirror element mounted therein. A thin, projecting side wall or edge of the case, extending about the entire periphery of the mirror elements therein is curled into an arcuate, peripheral, retaining edge. That edge defines a space extending between the entire periphery of the mirror and the case and allows the mirror and case to expand or contract in extreme temperatures without expelling or breaking the mirror element.
Production of mirror assemblies of the type as discussed above, wherein portions of one part must be heated and formed consistently and efficiently on a high volume basis, is the principal purpose of the present apparatus and method. We have discovered that for such high volume production, the projecting edge of the thermoplastic case must be heated precisely to a temperature within the transition temperature range of the material from which the case is formed. The transition temperature range is that in which the material softens, undergoes large changes in mechanical strength, and undergoes large changes in viscosity. Such physical transition is analogous to the changes which occur in glass which are defined by glass transition or transformation temperatures. The case including the heated portions must be transferred quickly and smoothly such that the heated portions may be formed around the edge of the reflective mirror element before the heated material cools sufficiently below the transition temperature range and regains its memory. All of these steps must be taken with the case and mirror elements being held in exact alignment and must be nearly exactly reproduceable in order to maintain consistently high quality mirror assemblies. Moreover, these steps must be repeated at a sufficiently high rate of speed to produce a high volume of mirror assemblies.